1. Field of the Invention
The present invention relates to a surface acoustic wave device, particularly a surface acoustic wave device using a unidirectional interdigital transducer (referred to as IDT hereafter).
2. Description of Related Art
Conventionally, unidirectional IDTs which excite surface acoustic waves (referred to as SAW hereafter) only in one direction have been developed in order to reduce loss of IDTs which excite the SAW going forward in two directions. Among such unidirectional IDTs, unidirectional IDTs of internal reflection type, which do not need external phase circuits and can be produced by forming an electrode film and patterning only once, are of practical use and the development thereof is active.
FIG. 15 shows a unidirectional IDT of internal reflection type using a floating electrode (disclosed by Japanese Unexamined Patent Publication No. SHO 60(1985)-236312).
Here, the unidirectional IDT is realized by placing an open floating electrode and a short-circuit floating electrode between a positive excitation electrode and a negative excitation electrode. In this unidirectional IDT of internal reflection type, the excitation electrodes and the floating electrode are .lambda./12 in width (.lambda. is an electrode period, i.e., a wavelength at a center frequency at which the SAW is excited most intensively), and the electrodes are apart from each other by a center-to-center distance of .lambda./6.
Further, Japanese Unexamined Patent Publication No. HEI 3(1991)-133209 discloses that directivity is improved by rendering the width of the open floating electrode more than .lambda./12 and that of the short-circuit floating electrode less than .lambda./12. In this unidirectional IDT with the floating electrodes, however, the width of the excitation electrodes is as narrow as .lambda./12, and accordingly there is a problem in that the production of IDTs becomes more difficult as the frequency increases.
Further, an edge-to-edge distance between the positive and negative excitation electrodes is 5.lambda./12. This is larger than an edge-to-edge distance of .lambda./4 in the case of a single-electrode IDT shown in FIG. 16 and an edge-to-edge distance of .lambda./8 in the case of an IDT with split electrodes shown in FIG. 17. Accordingly, there is a problem in that input impedance becomes greater.
In a unidirectional IDT disclosed in Japanese Unexamined Patent Publication No. SHO 61(1986)-6917, a positive electrode and a negative electrode are each composed of plural pairs of electrode fingers, the positive electrode finger pairs and the negative electrode finger pairs are alternately arranged, and the electrode fingers of each electrode finger pair are different in width, as shown in FIG. 18. Thus, the unidirectional IDT is constructed as a bilaterally asymmetric IDT. In this construction, the number of electrode fingers per period is four, and the construction is characteristic of being simpler compared with the aforesaid construction using the floating electrodes. Furthermore, the edge-to-edge distance between the positive and negative excitation electrodes is smaller than in the case of the IDT with split electrodes shown in FIG. 17. Accordingly, the input impedance is decreased.
The above-described Japanese Unexamined Patent Publication No. SHO 61(1986)-6917 states that, in the unidirectional IDT shown in FIG. 18, the SAW is excited strongly in a direction indicated by an arrow 5 in the figure. Similarly, Japanese Unexamined Patent Publication No. HEI 8(1996)-288780, also referring to the unidirectional IDT shown in FIG. 18, describes that, if the wider electrode finger of the electrode finger pair is 3.lambda./16 wide and the narrower electrode finger of the electrode finger pair is .lambda./16 wide, the SAW is excited strongly in the direction indicated by the arrow 5 in FIG. 18.
On the basis of the description of these two publications, the inventors made a SAW filter using 36.degree. Y-X: quartz as a piezoelectric substrate, the unidirectional IDT shown in FIG. 18 as an input IDT and a bidirectional IDT with split electrodes as an output IDT, wherein the input IDT was placed in an orientation as shown in FIG. 19.
With reference to the description of Japanese Unexamined Patent Publication No. HEI 8(1996)-288780, the wider electrode finger of a split electrode of the unidirectional IDT was 3.lambda./16 wide and the narrower electrode finger thereof was .lambda./16 wide. The electrode period .lambda. of the IDT was 15 .mu.m. The electrodes were made of aluminum films of 0.2 .mu.m thick. The number of the electrode finger pairs was 95 in each of the input and output IDT.
In FIGS. 18 and 19, the reference signs "6" and "8" indicate the direction from the wider electrode finger toward the narrower electrode finger, and the reference sign "7" indicates a direction in which the SAW excited by the unidirectional IDT propagates toward the receiver IDT.
FIG. 20 shows a filter characteristic of this filter. There exists a great ripple around the center frequency (209 MHz to 210 MHz). In the case of the 36.degree. Y-X: quartz substrate, the construction of FIG. 19 does not bring a good filter characteristic. Further, the width of the wider electrode of the unidirectional IDT shown in FIG. 18 was varied for seeking a better filter characteristic, but a good filter characteristic without ripples is not obtained.
Thus, in the case of the 36.degree. Y-X: quartz substrate, there is a problem in that a great ripple appears around the center frequency when the unidirectional IDT is placed in the orientation shown in FIG. 19. In other words, the problem was that good filters cannot be produced even by placing the unidirectional IDT in the orientation shown in FIG. 19 by reference to the directivity described in Japanese Unexamined Patent Publication Nos. SHO 61(1986)-6917 and HEI 8(1996)-288780, depending on materials for the substrate.